on a diet = 节食减肥

比如：别人给你吃的

I'm good. Thanks. I'm on a diet. 不用了，谢谢。我在节食减肥。

lose weight

1. 减肥，不限于节食，可以是各种方法，意思更宽泛

I'm good. Thanks. I'm trying to lose weight. 不用了，谢谢。我在节食减肥。

2. 瘦了，减掉肥肉

You look like you lost some weight. 你看起来瘦了哦。

例句：

I want to lose weight because this holiday season I've eaten way too much. 我想减肥，因为这个假期吃的实在太多了。

I've been on a diet for 3 weeks. And I lost 10 pounds! 我节食三周了，减掉了10磅！

Congratulations!

希望大家都减肥成功哦~

或者根本不肥

就像我…

I've been trying to lose weight for a year. Nothing happened.